UI: allow spreading widgets to shrink
UI: allow spreading widgets to shrink

Automatically shrinking a widget in the spread direction is needed, so the
spreading widget doesn't consume all space requested by non-spreading widgets
if the window is under-allocated.

Signed-off-by: Tim Janik <6ec6ff229c4e1c5bbd5933ee92e6c5832ff023bb@gnu.org>

tim-janik/rapicorn,tim-janik/rapicorn,tim-janik/rapicorn,tim-janik/rapicorn,tim-janik/rapicorn